Volunteers in the newly formed “Peace Brigades” participate in a parade near the Imam Ali shrine in the southern holy Shiite city of Najaf, Iraq, Friday, after calls by the radical Shiite cleric Muqtatda al-Sadr to form brigades to protect Shiite holy shrines against possible attacks by Sunni militants.

BAGHDAD — The most respected voice for Iraq’s Shiite majority on Friday joined calls for the country’s prime minister to form an inclusive government or step aside, a day after President Barack Obama challenged Nouri al-Maliki to create a leadership representative of all Iraqis.

Grand Ayatollah Ali al-Sistani’s thinly veiled reproach was the most influential to place blame on the Shiite prime minister for the nation’s spiraling crisis.

The focus on the need to replace al-Maliki comes as Iraq faces its worst crisis since the withdrawal of U.S. troops in 2011. Over the past two weeks, Iraq has lost a big chunk of the north to the al-Qaida-inspired Sunni militants of the Islamic State of Iraq and the Levant, whose lightning offensive led to the capture of Mosul, the nation’s second-largest city.

The gravity of the crisis has forced the usually reclusive al-Sistani, who normally stays above the political fray, to wade into politics, and his comments, delivered through a representative, could ultimately seal al-Maliki’s fate.

Calling for a dialogue between the political coalitions that won seats in the April 30 parliamentary election, al-Sistani said it was imperative that they form “an effective government that enjoys broad national support, avoids past mistakes and opens new horizons toward a better future for all Iraqis.”

Deeply revered by Iraq’s majority Shiites, al-Sistani’s critical words could force al-Maliki, who emerged from relative obscurity in 2006 to lead the country, to step down.

On Thursday, Obama stopped short of calling for al-Maliki to resign, but his carefully worded comments did all but that. “Only leaders that can govern with an inclusive agenda are going to be able to truly bring the Iraqi people together and help them through this crisis,” Obama declared at the White House.

The Iranian-born al-Sistani, believed to be 86, lives in the Shiite holy city of Najaf, south of Baghdad, where he rarely ventures out of his modest house on a narrow alley near the city’s Imam Ali shrine and does not give media interviews. His call to arms last week prompted thousands of Shiites to volunteer to fight against the Sunni militants who now control a large swath of territory astride both sides of the Iraq-Syria border.

The extent of al-Sistani’s influence was manifested in the years following the 2003 U.S. invasion of Iraq when he forced Washington to modify its blueprint for the country and agree to the election of a constituent assembly that drafted the nation’s constitution.

For the past two years, he has shunned politicians of all sects, refusing to receive any of them to show his disillusionment with the way they run the country. However, the danger posed by the Islamic State militants appears to have forced him to say more.

His call to arms has given the fight against the Sunni insurgents the feel of a religious war between Shiites and Sunnis. His office in Najaf dismissed that charge, with his representative, Ahmed al-Safi, saying Friday: “The call for volunteers targeted Iraqis from all groups and sects. ... It did not have a sectarian basis and cannot be.”

Al-Maliki’s State of Law bloc won the most seats in the April vote, but his hopes to retain his job are in doubt with rivals challenging him from within the broader Shiite alliance. In order to govern, his bloc must first form a majority coalition in the new 328-seat legislature, which must meet by June 30.